1. Field of the Invention
This invention relates to a vibration wave driven motor in which a vibration member and a member being in pressure contact with the vibration member are moved relative to each other by a travelling vibration wave generated in the vibration member, and more particularly to the material of a sliding member in the pressure contact portion.
2. Related Background Art
The outline of the principle of a vibration wave driven motor utilizing a travelling vibration wave is as will be described below.
Two groups of piezo-electric elements arranged circumferentially on and secured to one surface of a ring-like vibration member of a resilient material whose full circumferential length is integer times a certain length .lambda. is defined as a stator. These piezoelectric elements as electro-mechanical energy conversion elements, in each groups, are arranged at a pitch of .lambda./2 and so as to be alternately opposite in expansion and contraction polarity, and all arranged so that there may be deviation odd number times as great as .lambda./4 between the two groups. The two groups of piezo-electric elements have electrode films applied thereto. If an AC voltage is applied only to one group (hereinafter referred to as the A phase), a standing wave (wavelength .lambda.) of such fluxural vibration that the midpoint of each piezo-electric element in each group and points at every .lambda./2 therefrom are the positioned of loops and the midpoint between said positioned of loops is the position of mode is generated over the full circumference of said vibration member. If an AC voltage is applied only to the other group (hereinafter referred to as the B phase), a standing wave is likewise generated, but the positions of the loops and node deviate by .lambda./4 from those of the standing wave by the A phase. If AC voltages identical in frequency to each other and having a time phase difference of 90.degree. therebetween are applied to the A and B phases at a time, the standing waves of the two are combined together with a result that a travelling wave (wavelength .lambda.) of flexural vibration vibrating circumferentially is generated in the vibration member and at this time, each polygonal point of said vibration member having a thickness effects a kind of elliptical motion. Consequently, if for example, a ring-like moving member as a rotor is brought into pressure contact with each other surface of the vibration member, said moving member receives circumferential friction from the vibration member and is rotatively driven thereby.
Accordingly, it is desirable for taking out the output efficiently that a sliding material of high coefficient of friction be provided in the pressure contact portion between the vibration member and the moving member, and in the prior art, as described, for example, in Japanese Laid-Open Patent Application No. 62-100778, a mixture (WC-Co) of tungsten carbide and cobalt is used as a sliding material on the vibration member side and hard alumite is used as a sliding material on the moving member side.
Now, where such sliding materials are used, abrasion is small if the output of the vibration wave driven motor is 1 W or less, but if the pressure force of the vibration member and the moving member is made great or the number of revolutions is increased to obtain an output of the order of 3-5 W, it has led to a problem that the abrasion of the hard alumite layer which is the sliding material on the moving member side progresses rapidly and at a point of time whereat this layer has disappeared, the reduction in the torque performance of the vibration wave driven motor becomes remarkable and the like of the vibration wave driven motor comes to an end.
When the state of this abrasion has been examined, it has been found that the hard alumite layer has been fragility-destroyed.
Therefore, in order to increase the toughness of the sliding material on the moving member side, nylon 66 (hereinafter referred to an PA 66, the coefficient of friction: 0.8-1.5) which is thermoplastic resin has been used as the sliding material on this side and WC-Co has been used as the sliding material on the vibration member side and the motor has been driven, but the resin simple substance has produced a great deal of abrasion powder and stable rotation has not been achieved with such abrasion powder rolled in the sliding surface.
When the cause of this abrasion has been examined, it has been found that the cause is the shear peeling (adhesion abrasion) due to the adhesion of the two surfaces of resin and WC-Co which are the sliding materials on the two sides. To prevent this adhesion abrasion, it is effective and popular to compound fiber and powder having slidability.
So, when PA 66 (coefficient of friction 0.4-0.6) having 10-30% by weight of high strength type carbon fiber compounded as a filler was used and the motor was driven, abrasion decreased sharply, but there occurred a phenomenon of torque falling sharply during constant speed rotation (hereinafter referred to as the torque down). When the resin sliding surface was observed after the driving, it was found that part of the compounded carbon fiber was lost. Judging from the manner in which carbon fiber was lost and the face that the coefficient of friction of carbon fiber is lower than that of PA66, it can be conjectured that carbon fiber was subjected to vibration and fragility-destroyed and abraded, and the torque down can be presumed to have caused the abraded powder of carbon fiber to be rolled into the sliding surface.
Also, in any of the above-described cases, there occurred sometimes abnormal sounds.
That is, summing up what has been described above, there are the following difficulties:
(1) The lack of toughness of the sliding material causes the sliding material to be fragility-destroyed and abraded;
(2) If the sliding material is a resin simple substance, adhesion abrasion is caused; and
(3) If a filler is put into resin to solve the problem mentioned under item (2) above, the adhesion abrasion of resin decreases, but the lack of toughness of the filler itself causes the filler to be fragility-destroyed and abraded under the influence of vibration; and these difficulties have caused irregular torque, torque down and occurrence of abnormal sounds in the performance of the viabration wave driven motor.